The production of oxygen from air, using cryogenic methods, is both capital and power intensive. Presently, standard double column-type air separation plants are commonly used for the production of moderate purity oxygen (85% to 98%). With the improvement of non-cryogenic technologies (such as adsorption), there is an acute and growing need to reduce both power consumption and capital cost of cryogenic plants at this level of oxygen purity. A dual-dephlegmator cycle (i.e., rectification and stripping cycle) offers the potential to reduce power but may not reduce capital cost unless it is implemented effectively. It is the object of this invention to provide a process/apparatus which provides savings in both capital and power.
Numerous dephlegmator processes are known in the art; among these are the following:
U.S. Pat. No. 2,861,432 discloses a dual-dephlegmator cycle for oxygen production. The most relevant embodiment of that invention is illustrated in FIG. 1. The key features of U.S. Pat. No. 2,861,432 are as follows (identifier numbers correspond to FIG. 1): A high pressure rectification dephlegmator (23) accepts chilled feed air at the bottom (28) and produces enriched nitrogen vapor as overhead (25) and crude liquid oxygen as bottoms (32). A low pressure stripping dephlegmator (24) accepts a liquid flowing from the fractionating column (21) at the top, produces enriched oxygen as a liquid bottoms product (26), and rejects vapor out the top which flows up into the fractionating column (21). The rectifying and stripping dephlegmators are in thermal contact to facilitate heat exchange. A high pressure condenser (34) converts the rectification dephlegmator overhead (25) from vapor to liquid (this liquid is used as top reflux to the fractionating column (21)). This condenser consists of tubes (34) immersed in liquid in the column (21). The fractionating column (21), which carries-out both rectification and stripping, is also present. Boilup for the column is provided by vaporizing some of the liquid on the lower trays. The heat transfer device used is of the tube type (34). The heat for vaporization comes from the heat rejected by the condensation of rectification dephlegmator overheads. This column accepts the enriched, liquid nitrogen from the high pressure condenser as the top most feed, liquid air (31) as an intermediate feed, crude liquid oxygen from the high pressure dephlegmator (32) plus expander air (41) as the third feed. Vapor rejected from the low pressure stripping dephlegmator flows into the lower trays. The liquid from the fractionation column is the feed to the low pressure stripping dephlegmator while the overheads is a nitrogen enriched "waste" stream (42). The liquid air feed is produced by vaporizing the liquid oxygen product from the bottom of the low pressure stripping dephlegmator. The vaporization/condensation takes place in a separate exchanger (27). Two (2) pressure levels of air enter the plant. Eighty percent (80%) of the air enters at the lower pressure (at about 60 psia). After chilling, the lower pressure feed is split into two streams. Essentially, half of this flow is expanded to provide refrigeration, the other half is sent to the rectification dephlegmator. Twenty percent (20%) of the air enters at a higher pressure (at about 70 psia) and is condensed against boiling oxygen product. The pressure of the oxygen product is near atmospheric pressure.
U.S. Pat. No. 2,861,432 also discloses an apparatus which is assembled with a material called overflow packing. The apparatus, which could be used to combine the stripping and rectification dephlegmator functions, is contained within the low pressure distillation column with the stripping dephlegmator side open to the column and the other enclosed. A further discussion of overflow packing is disclosed by Winteringham et al, in an article in Trans Instn Chem Engrs, page 55, Vol 44, 1966.
Despite the foregoing, there are numerous disadvantages associated with the teachings of U.S. Pat. No. 2,861,432; among these are the following: Overflow packing has limited vapor capacity and low mass transfer/heat transfer efficiency because so much liquid is held-up. The "packing unit" is inserted within the column itself which represents poor use of volume (a rectangular device in a circular container). The overflow packing is inappropriate for oxygen service because it presents a series of liquid accumulation points for hydrocarbons to concentrate. Furthermore, the use of tubes-immersed-in-liquid to operate the reflux condenser (34) is a mechanically complex proposition.
U.S. Pat. No. 4,025,398 discloses a process and (primarily) various devices for heat integrating rectification and stripping sections of distillation columns with heat exchange equipment running between individual distillation stages of two columns.
U.S. Pat. No. 3,756,035 discloses a process wherein separation takes place in a plurality of fractionating zones with the respective fractionating zones being connected in adjacent side-by-side indirect heat exchange relation with one another. U.S. Pat. No. 3,756,035 also discloses that the fractionating passages can be channels bearing the liquid-vapor mixture being separated in the column. Such channels may be constructed in a manner of a perforated fin compact heat exchanger, producing the effect of distillation column trays. This type of heat exchanger arrangement is also described in International Advances in Cryogenics, Vol. 10, pp 405, 1965. Though the reference is somewhat vague, it is believed to be referring to overflow packing.
U.S. Pat. No. 4,308,043 also relates to partial heat integration of rectification and stripping sections.
U.S. Pat. No. 4,234,391 discloses a method and apparatus which thermally links the stripping and rectifying sections of the same column. The apparatus consists of a trayed column with a wall running down the centerline and heat exchange tubes which transfer energy from one tray to another.
U.S. Pat. No. 3,568,461 discloses a fractionating apparatus using serrated fins for use in adiabatic or differential distillation.
U.S. Pat. No. 3,568,462 also discloses a fractionating apparatus made from perforated fin in the hardway flow orientation.
U.S. Pat. No. 3,612,494 discloses a gas-liquid contacting device using a plate-fin exchanger. U.S. Pat. No. 3,992,168 discloses a means of vapor-liquid distribution for plate-fin fractionating devices.
U.S. Pat. No. 3,983,191 describes the use of plate-fin exchanger for non-adiabatic rectification.
U.S. Pat. No. 5,144,809 discloses a rectification dephlegmator for nitrogen production using a plate-fin exchanger. There is no stripping dephlegmator. The dephlegmator produces nitrogen at, essentially, feed air pressure. Crude liquid oxygen is boiled against the dephlegmating nitrogen such that no separation is performed on the crude liquid oxygen.
U.S. Pat. No. 5,207,065 also discloses a rectification dephlegmator for nitrogen production based on a plate-fin heat exchanger.
Finally, U.S. Pat. No. 5,410,855 discloses a double column cryogenic rectification system wherein the lower pressure column bottoms undergo additional stripping within a once-through downflow reflux condenser by countercurrent direct contact flow with vapor generated by condensing higher pressure column shelf vapor.